In various types of semiconductor photo detectors, p-i-n photodiode (hereinafter referred to as “PIN photodiode”) is well known. This device is capable of detecting photon as photoelectric current that is generated by the following processes, in which an electric field is applied over a low concentration layer of a semiconductor (I layer) sandwiched with a p-type layer and an n-type layer to generate photo carrier in the I layer, which travels therethrough to create photoelectric current.
On the other hand, photo detectors exhibiting higher sensitivity are more useful in the field of optical communications and optical measurements. Devices referred to as avalanche photodiodes (hereinafter, abbreviated as “APD”), which utilize avalanche effect generated by applying strong electric field into a semiconductor, are often employed. A multiplier layer having higher internal electric field is included in a layer configuration of an APD, and ionization is caused in such multiplier layer in a manner similar to chain reaction to amplify photocarrier. Since the noise generated during such amplification process is a noise caused by a shot noise as represented by 2qIM2F, such amplification may be achieved, so that such noise is comparable with the thermal noise, to provide an enhanced sensibility of the detector.
The materials and the device structures of the APD may be generally classified by substrate materials (silicon (Si), germanium (Ge), gallium arsenide (GaAs), indium phosphide (InP) or structures (planar-structure and mesa-structure). Planar-structure elements formed on indium phosphide (InP) substrates are often employed in recent years for the APD employed in the area of the optical communications. This is because such type of elements is capable of detecting light having a wave length λ=1.55 μm, which is utilized in the optical fiber communication, such that it is convenient that indium gallium arsenide (InGaAs), which is in lattice match with an InP substrate, would be employed for optical absorption layer. The use of InGaAs allows receiving light having a wave length of up to 1.6 μm at a room temperature. Another reason for the frequent use of the planar structure is that the use of such structure allows achieving enhanced long term-reliability, which is required for the optical communications.
It is known that the mesa-structure device provides easier manufacture but generally reduced long term-reliability, as compared with the planar-structure device.
A typical conventional example of a mesa-structure APD element is shown in FIG. 6. Such conventional mesa-structure APD element is configured that an n-type field-relaxing layer 603, a multiplier layer 604, a p-type field-relaxing layer 605, a p-type light absorption layer 606, a p-type buffer layer 608 and a p-type contact layer 609 are sequentially deposited on a semiconductor substrate 601, and the structure is formed by an etching process to provide the whole p-n junction as a mesa, and in addition, a passivation film 612 is formed outside thereof. A p-type electrode 610 and an n-type electrode 611 are respectively formed, and then a polishing process, a process for forming an anti-reflection film (AR coating), and a process for achieving an element isolation are carried out to produce a front surface incident-type or a back surface-incident type diode for a semiconductor photo detection.
Related literatures include the following literatures.    [Non-Patent Document 1]    Watanabe, I., et al., JOURNAL OF LIGHT WAVE TECHNOLOGY, vol. 15, No. 6, June 1997, p. 1012, entitled “Design and Performance of InAlGaAs/InAlAs Superlattice Avalanche Photodiodes”, and    [Patent Document 1]    Japanese Patent Unexamined Publication No. 2005-539368.